Di-tertiary-alkylation of aromatic compounds produces almost exclusively the p,p'-isomer due to stereoelectronic effects of the bulky tertiary-alkyl (t-alkyl) group. However, aromatic compounds with t-alkyl substituents are difficult to oxidize to the acids. As reported in Sheldon and Kochi, Metal-Catalyzed Oxidations of Organic Compounds, "Alkyl aromatic compounds without benzylic C-H bonds (e.g., t-butylbenzene) are, of course, unreactive toward auto-oxidation." Examples in U.S. Pat. Nos. 2,833,816 and 3,089,907, which disclose the oxidation of alkyl aromatics, clearly show that t-butyl groups are resistant to oxidation.
Auto-oxidation of alkyl aromatics to produce aromatic acids was disclosed in a series of five patents issued in 1958, Saffer et al., U.S. Pat. Nos. 2,833,816-20. These patents taught the use of heavy metal carboxylate salts as catalysts, in particular, manganese carboxylates. In U.S. Pat. No. 2,833,816, the disclosed catalyst contains a heavy metal or mixture of metals (manganese, cobalt, nickel, chromium, vanadium, molybdenum, tungsten, tin or cerium, preferably manganese or cobalt), an aliphatic acid of 1-8 carbons, and a source of bromine to serve as a promoter. The metal can be provided as the metal, as a metal complex or as a salt, with the preferred form being as the salt of an aliphatic carboxylic acid. Mixed metal catalysts are exemplified by a mixture containing one part cobalt and one-to-three parts manganese. The bromine promoter can be supplied as elemental bromine, hydrobromic acid, an ionic bromide salt, or an organic bromine-containing compound. The metal bromide salt, which may be added directly or formed during the reaction from the above sources of these components, should be present at 0.1-10% by weight, based on the concentration of the aromatic reactant. The aromatic compounds which may be oxidized in the presence of this catalyst contain alkyl groups attached through either primary or secondary alkyl carbons. U.S. Pat. No. 3,089,907, a continuation-in-part of the application that became U.S. Pat. No. 2,833,816, shows that t-butyl groups attached to an aromatic ring are not oxidized in this system, even when methyl groups attached to the same ring are being oxidized to carboxyl groups. A specific example is given of oxidation of t-butyl-m-xylene to t-butyl-isophthalic acid.
Holz, in J. Org. Chem., volume 37, pages 2069-74 (1972), studied the oxidation of alkyl-substituted benzene and reported that t-butyltoluene could be oxidized by molecular oxygen to terephthalic acid by using cobalt chloride as catalyst instead of cobalt bromide; however, the yield of terephthalic acid was very low. The catalyst used in this study was cobalt acetate with hydrochloric acid in a mixed solvent of acetic acid and chlorobenzene.
Para,para'-dicarboxypolyphenyls (p,p'-dicarboxypolyphenyls) are of great interest to the art of high performance liquid crystal polymers. For example, replacing terephthalic acid with 4,4'-dicarboxybiphenyl or 4,4'-dicarboxy-p-terphenyl imparts greater rigidity and stability to polyester polymers, with resulting improvement in performance characteristics. However, up to now such benefits have been unavailable in the art, because an environmentally safe, economical method for the preparation of the p,p'-dicarboxypolyphenyls was lacking.
A number of methods for preparation of 4,4'-dicarboxybiphenyl have been described in the prior art, although none of them have found commercial application. UK Patent 2,155,921 teaches carboxylation of 4-alkylbiphenyl in the presence of HF/BF.sub.3 followed by oxidation of the alkyl group. This process has the disadvantage of requiring stoichiometric amounts of an expensive component, HF. Another expensive route is taught in Japanese Patent Application JP 57/149,243, now issued as JP 83/46,494. In this patent, 4,4'-dicarboxybiphenyl is formed from the dipotassium salt of diphenic acid which can be isomerized with Cd catalysts at high temperature in the presence of carbonic acid gas, but with only low-to-moderate yields. Another unsatisfactory method, diacylation of biphenyl with acetyl chloride, as disclosed in U.S. Pat. No. 3,383,402, requires stoichiometric amounts of AlCl.sub.3.
Methods also have been disclosed for the production of 4,4'-dicarboxybiphenyl from halogenated aromatic compounds. EP 0,206,543 teaches the coupling of p-chlorobenzoic acid, while U.S. Pat. No. 3,636,082 teaches carboxylation of 4,4'-dibromobiphenyl. Unfortunately, both methods have the potential to generate halogenated biphenyls as undesirable by-products.
U.S. Pat. No. 3,296,280 discloses the oxidation of 4-t-butyl-4'-carboxybiphenyl by NO.sub.2 at high temperatures to produce 4,4'-dicarboxybiphenyl, but the examples show that the reaction proceeds only in low yield and produce undesirable nitrated by-products. Oxidation of 4,4'-dimethylbiphenyl by NO.sub.2 has been reported in U.S. Pat. No. 3,631,097, but there is currently no economical route to 4,4'-dimethylbiphenyl. All economical syntheses of methyl-substituted biphenyl produce a mixture of isomers with a low yield of the p,p'-isomer.
A need remains for an improved process to oxidize t-alkyl aromatics to acids and to produce para-oriented aromatic acids.